Plant growth in India depends almost entirely on the strength of the annual monsoon. When the monsoon rains fail, so too do the country's crops. Good rains provide bumper crops. Beyond controlling the fate of agriculture in India, changes in the Indian monsoon helped scientists recognize the far-flung impact of the oscillating Pacific Ocean phenomena El Nino and La Nina.
It was while puzzling out patterns of drought in India in the early twentieth century that Sir Gilbert Walker first noticed a change in air pressure in the South Pacific that seemed to influence the Indian monsoon. Years later, in the 1960s, Jacob Bjerknes was also studying drought in India when he connected the changes in air pressure that Walker had noticed to changes in ocean temperatures known as El Nino and La Nina. Bjerknes realized that the periodic warming and cooling of the Pacific Ocean occurred as changes in the air pressure strengthened or weakened trade winds; these changes altered weather patterns around the world.
La Nina occurs when strong trade winds cool the equatorial Pacific Ocean in the east off the coast of South America, while allowing warmer water to build in the west near the Philippines and Indonesia. Its counterpoint, El Nino warms the equatorial Pacific by weakening trade winds. La Nina dominated the Pacific throughout 2007 and into 2008. (See earthobservatory.nasa.gov/Newsroom/NewImages/images.php3 La Nina and Pacific Decadal Oscillation Cool the Pacific on the Earth Observatory.) Though La Nina doesn't directly control monsoon rains in India, it does influence them. La Nina tends to bring abundant monsoon rains to India, and the 2007 monsoon season was accordingly above-average. By early April 2008, plants throughout the country were responding to the plentiful water supply.
NASA image created by Jesse Allen, using data provided by the United State Department of Agriculture Foreign Agriculture Service and processed by Jennifer Small, NASA GIMMS Group at Goddard Space Flight Center. Caption by Holli Riebeek.